U.S. Pat. No. 4,181,694 teaches a process for manufacturing hollow fibers from acrylonitrile polymers, with an acrylonitrile polymer composed of at least 60 wt. % acrylonitrile units being dissolved in an aqueous 65 to 95 wt. % nitrating acid (at 0.degree. to 5.degree. C.), to obtain a spinning solution. This solution is then extruded through spinnerets and an internal coagulation fluid is introduced at the same time. This process yields hollow fibers composed of an outer layer with a porous structure, a middle layer with so-called caverns, and an inner layer with a porous structure. These are doubly asymmetric membranes with a skin on both sides. The term "caverns" refers to "relatively large" holes in the membrane wall.
By analogy with the above patent, DE-PS 27 40 252 claims dried porous membranes with acrylonitrile polymer units composed of a carrier layer and a bilateral surface layer. A process for manufacturing this membrane having a skin on both outer surfaces is also claimed.
U.S. Pat. No. 4,545,910 claims membranes that exhibit the performance data of a conventional ultrafiltration membrane. The material for the membrane can be chosen from a plurality of materials, including polyacrylonitrile compounds.
Recently, U.S. Pat. No. 5,039,420 claims a membrane consisting of a copolymer of acrylonitrile and a C.sub.2 -C.sub.4 hydroxyalkyl ester of methacrylic acid as comonomers.
However, the membranes manufactured according to the above processes exhibit only unsatisfactory values in certain areas as far as biocompatibility is concerned, for example in histamine release or bradykinin generation.
In the manufacture of synthetic, noncellulose membranes, for example those made of materials such as polyether sulfone, polyamide, or polyacrylonitrile compounds, a number of properties of the material that play a role in the future application of the membrane must be taken into account.
Therefore, a membrane of this kind, if it is to be used for dialysis, should exhibit or produce histamine release which is as low as possible. Increased histamine release in dialysis patients results in a number of unpleasant side effects, such as headache and pains in the limbs, as well as other painful conditions that have a negative effect on the state of health of the patient. Of course the limiting value for histamine release must be determined individually for each person. This value depends on a plurality of factors (age, sex, weight, etc.) and therefore cannot be expressed in general terms.
Histamine is a biologically highly active substance. Therefore, its excessive release must be avoided at all costs. See the papers by E. Neugebauer et al., Behring Inst. Mitt., No. 68, pp. 102-133, 1981 and W. Lorenz et al., Klin. Wochenschr. Vol. 60, p. 896-913, 1982.
In addition, a membrane of this kind should exhibit values that are as low as possible for bradykinin generation. Bradykinin generation is also associated with unpleasant side effects that could pose a risk to dialysis patients. (G. Bonner et al., J. of Cardiovasc. Pharm. Vol. 15 (Suppl. 6), pp. 46-56, 1990). Although the clinical significance of bradykinin generation like that of histamine release has not yet been completely studied, an attempt should be made to avoid bradykinin generation whenever possible during dialysis, as it is triggered by a large quantity of sulfonate compounds in the membrane as a result of so-called "contact activation."